Generally, a raindrop detection apparatus is provided in a wiper control device for a vehicle to detect raindrop landed on a windshield of the vehicle, for example, referring to JP-A-2001-521158 (U.S. Pat. No. 5,898,183) and JP-A-2001-66246 (U.S. Pat. No. 6,507,015).
The raindrop detection apparatus has a first lens unit that converts light from a light emitting unit into parallel light, a light conducting unit that conducts the parallel light to the windshield and conducts light reflected by an outer surface of the windshield, and a second lens unit that converges reflection light at a light receiving unit.
In this case, light from the light emitting unit passes through the interior of the windshield and is reflected at the outer surface of the windshield, then received by the light receiving unit. The raindrop detection apparatus is designed according to the thickness of the windshield on which the first lens unit, the second lens unit, and the light conducting unit are mounted, so that the light receiving unit can properly receive the light from the light emitting unit.
Recently, the wiper control device is provided for not only a large-sized vehicle but also a small-sized vehicle, to improve a traveling safety even in a rainy weather. Generally, the thickness of the windshield differs according to the vehicle types. For example, the small-sized vehicle has a thinner windshield than the large-sized vehicle. Thus, the raindrop detection apparatus is to be designed responding to the thickness of the windshield, at which the first lens unit, the second lens unit, and the light conducting unit are mounted. Therefore, the manufacturing cost is increased.
Moreover, referring to JP-A-2001-66246, the raindrop detection apparatus is provided with the first lens unit (at side of light emitting unit) that includes multiple small-diameter lenses with a gap arranged therebetween. The first lens unit has a shape and a construction significantly different from those of the second lens unit (at side of light receiving unit), thus making the optical axis design complex. Furthermore, in this case, light emitted to the gap is not used (not received by light receiving unit) to be wasted.